Optical waveguide and method for its manufacture

ABSTRACT

An optical waveguide is made by forming a layer of SiO 2  on a substrate and implanting a region of the SiO 2  layer with Si ions to define a region containing a stoichiometric excess of Si which defines a region having an elevated refractive index surrounded by a region having a lower refractive index. The resulting optical waveguide is stable at the high temperatures required for many semiconductor processing methods, and is useful for optical interconnection in integrated optical and optoelectronic devices.

FIELD OF THE INVENTION

This relates to optical waveguides and methods for their manufacture.

BACKGROUND OF THE INVENTION

Planar optical waveguides are required for optical interconnection inintegrated optical and optoelectronic devices. The methods used tomanufacture such waveguides must be compatible with semiconductorprocessing methods used to manufacture other parts of the integrateddevices.

Planar optical waveguides have been made by depositing a photosensitivemonomer on a substrate and selectively exposing the deposited monomer toultraviolet (UV) radiation. The UV radiation polymerizes the exposedmonomer to provide polymer regions having a relatively high refractiveindex bounded by monomer regions having a relatively low refractiveindex. A further monomer layer is generally deposited over the partiallypolymerized layer for protection against surface flaws and contaminantswhich could couple light out of the polymerized regions. Unfortunately,the waveguides made by this method are unstable at the high temperatureswhich are used in some semiconductor processing methods. Consequently,all high temperature processing steps must be completed before thewaveguides are defined. Moreover, this method generally requires two ormore deposition steps.

Planar optical waveguides have also been made by depositing or growing afirst layer of SiO₂ on a substrate, depositing a layer of Si₃ N₄ on thefirst layer of SiO₂, depositing a second layer of SiO₂ on the Si₃ N₄layer, and selectively removing a partial thickness of the second SiO₂layer in selected regions to lower the effective refractive index of theunderlying Si₃ N₄ layer in those regions. This method requires threedeposition or growth steps and one etch back step, all of which must becarefully controlled for satisfactory results.

Silicon-based planar optical waveguides have also been made bydepositing or growing a first layer of undoped SiO₂ on a substrate,depositing P-doped SiO₂ on the layer of undoped SiO₂, selectivelyremoving regions of the P-doped SiO₂ layer to expose regions of thefirst layer of undoped SiO₂, and depositing a second layer of undopedSiO₂ on the exposed regions of the first layer of undoped SiO₂ and onthe remaining regions of P-doped SiO₂. The regions of P-doped SiO₂ havea higher refractive index than the surrounding regions of undoped SiO₂.This method also requires three deposition or growth steps and one etchback step, all of which must be carefully controlled for satisfactoryresults.

In U.S. Pat. No. 4,585,299, Robert J. Strain discloses a method formaking silica-based planar optical waveguides in which boron,phosphorus, arsenic or germanium is implanted into a silicon substratethrough a first mask and the substrate is oxidized through a second maskto provide a patterned SiO₂ layer which incorporates the implanteddopant. The implanted dopant raises the refractive index of a centralregion of the SiO₂ layer to provide a waveguide. This patent suggeststhat migration of the dopant during the oxide growth may be a problem.

Silicon-based planar optical waveguides have also been made bydepositing or growing a layer of SiO₂ on a substrate and selectivelybombarding the SiO₂ with H or B ions to define regions having arelatively high refractive index bounded by regions having a relativelylow refractive index. The implantation process causes localizedcompaction of the SiO₂ which locally increases the refractive index ofthe Si)₂. The presence of the implanted H or B ions may also modify therefractive index of the implanted SiO₂. Unfortunately, the SiO₂ isdecompacted and the implanted H or B ions are redistributed by diffusionin the SiO₂ layer if the waveguides are subjected to subsequent hightemperature processing steps. The decompaction of the SiO₂ and themigration of the implanted H or B ions degrades the refractive indexprofile defined by the implantation process. Consequently, all hightemperature processing steps must be completed before the waveguides aredefined.

SUMMARY OF THE INVENTION

This invention seeks to obviate or mitigate problems with known planaroptical waveguides and methods for their manufacture as described above.

One aspect of the invention provides an optical waveguide comprising asubstrate and a layer of SiO₂ on the substrate. The layer of SiO₂comprises a region containing a stoichiometric excess of Si whichdefines a region having an elevated refractive index surrounded by aregion having a lower refractive index.

Another aspect of the invention provides a method for making an opticalwaveguide. The method comprises the steps of forming a layer of SiO₂ onthe substrate and implanting a region of the SiO₂ layer with Si ions todefine a region having an elevated refractive index surrounded by aregion having a lower refractive index.

The optical waveguide according to the invention is stable at the hightemperatures required for many semiconductor processing methods. Samplewaveguides were annealed at 1100 degrees Celsius in a non-oxidizingambient for 12 hours without loss of definition of the refractive indexprofile. However, high temperature processing in an oxidizing ambientdoes cause loss of definition of the refractive index profile.

The method according to the invention requires only a single depositionor growth step, and no etch back step. Consequently this method isrelatively simple and easy to control. Moreover, the method iscompatible with standard semiconductor processing methods, and can beperformed using readily available semiconductor processing equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below by way of example only.The description refers to the attached drawings, in which:

FIGS. 1a, 1b and 1c are cross-sectional views of an optical waveguideaccording to an embodiment of the invention at successive stages of itsmanufacture by a method according to a first embodiment of theinvention;

FIG. 2 is a plot of refractive index versus depth for the opticalwaveguide of FIG. 1; and

FIGS. 3a, 3b, and 3c are cross-sectional views of an optical waveguideaccording to an embodiment of the invention at successive stages of itsmanufacture by a method according to a second embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

In a method according to a first embodiment of the invention for makingan optical waveguide, a layer 10 of SiO₂ is grown by steam oxidation ofa <100> Si substrate 12 to form the structure shown in FIG. 1a. Thesteam oxidation is performed at 950 degrees Celsius and at atmosphericpressure to provide an SiO₂ layer approximately 710 nm thick.

A layer 14 of Si₃ N₄ approximately 2 microns thick is deposited on theSiO₂ layer, and defined using conventional photolithographic techniquesto provide an opening 16 through the Si₃ N₄ layer 14 where a waveguidechannel is desired. The resulting structure, shown in FIG. 1b, isinserted into conventional ion implantation equipment, where it issubjected to a dose of Si ions 18 at an implant dose of 4×10¹⁶ cm⁻² andan implantation energy of 40 keV. The Si₃ N₄ layer 14 acts as an ionimplantation mask to provide selective implantation of the Si ions 18into the SiO₂ layer 14 only through the opening 16.

The Si₃ N₄ layer 14 is removed using conventional techniques for theselective removal of Si₃ N₄ to leave the layer 10 of SiO₂ which nowcomprises an implanted region 20 containing a stoichiometric excess ofSi as shown in FIG. 1c. The stoichiometric excess of Si as a function ofdepth approximates a Gaussian distribution function. The refractiveindex of the implanted region 20 is elevated by the presence of theexcess Si in proportion to the local concentration of the excess Si.Thus, the excess Si defines a graded refractive index profile whichdefines a region having an elevated refractive index surrounded by aregion having a lower refractive index.

FIG. 2 illustrates the refractive index profile of the implanted opticalwaveguide which may be measured using conventional etch back techniquescombined with conventional ellipsometric refractive index measurements.

Waveguides made by methods similar to the method described above havebeen annealed in an inert ambient at 1100 degrees Celsius for 12 hourswithout detectable changes in the refractive index profile. Theseresults indicate that although a minor proportion of the refractiveindex increase may be due to compaction of the SiO₂, a mechanism whichis reversed at high temperatures, most of the refractive index increasemust be due to a different mechanism which is stable at hightemperatures. It is believed that the increased refractive index of theSi-implanted SiO₂ is primarily due to the formation of Si--Si bondswhich are stable at high temperatures. Thus, high temperaturesemiconductor processing steps which are conducted in an inert ambientmay follow the formation of waveguides by the above method withoutdegradation of the waveguide structure.

However, exposure of the implanted layers to high temperature processingin an oxidizing ambient reverses the refractive index increase due toimplantation, probably because the presence of excess oxygen at elevatedtemperatures disrupts Si--Si bonds formed during implantation to formfurther SiO₂. This effect can be used in an alternative method formaking an optical waveguide as described below.

In a method according to a second embodiment, an SiO₂ layer 10 is grownas in the first embodiment. The implantation masking Si₃ N₄ layer 14 ofthe first embodiment is omitted, and the entire SiO₂ layer 10 isimplanted with Si ions to form a refractive index profile, as shown inFIG. 3a. A layer 14 of Si₃ N₄ is then deposited on the SiO₂ layer 10 anddefined using conventional photolithographic techniques so that the Si₃N₄ layer 14 remains only over regions of the SiO₂ layer 10 where awaveguide is desired, as shown in FIG. 3b. The resulting structure isthen heated in an oxidizing ambient to oxidize the implanted Si inregions of the SiO₂ layer 10 which are not covered by the remaining Si₃N₄ layer 14 to erase the refractive index profile in those regions, asshown in FIG. 3c. The Si₃ N₄ layer 14 acts as an oxidation-resistantmask to prevent oxidation of the implanted Si and erasure of therefractive index profile in the regions where a waveguide is desired.

The methods described above may be modified by growing the SiO₂ layer 10on Si substrates of different orientations and at different temperaturesor pressures. Pressures exceeding atmospheric pressure may be requiredwhere a thick SiO₂ layer is desired. The SiO₂ layer may be formed on Sisubstrates or on substrates of materials such as III-V semiconductors byprocesses other than thermal growth such as chemical vapour deposition.

The SiO₂ thickness, the implantation energy and implantation dose may bemodified to change the depth and refractive index profile of theresulting waveguide. For example, the implantation energy may range from3 keV to 400 keV, and the implantation dose may range from 1×10¹⁴ cm⁻²to 2×10¹⁷ cm⁻².

Non-Gaussian refractive index profiles can be obtained by performing aseries of implantations at different implantation energies andoptionally at different implantation doses. Successive implantations canbe performed through different implantation masks to provide differentrefractive index profiles in different regions of the SiO₂ layer 10. Aseries of implantations through a common implantation mask can be usedto provide a high refractive index well which extends to the surface ofthe SiO₂ layer 10 for surface coupling of a waveguide to an opticalfiber or an optical device.

Other masking materials, such as polysilicon or Al may be used duringimplantation, and the thickness of the masking material should beselected to be at least three to five times the projected range of Siions in the selected masking material at the selected implantationenergy. These and other modifications are within the scope of theinvention as claimed below.

We claim:
 1. An optical waveguide, comprising:a substrate; and a layerof SiO₂ on the substrate, the layer of SiO₂ comprising a regioncontaining a stoichiometric excess of Si which defines a region havingan elevated refractive index surrounded by a region having a lowerrefractive index.
 2. An optical waveguide as defined in claim 1, whereinthe stoichiometric excess of Si defines a graded refractive indexprofile.
 3. An optical waveguide as defined in claim 2, wherein thestoichiometric excess of Si as a function of depth approximates aGaussian distribution function.